Water-soluble polymer powders are prepared by polymerizing water-soluble ethylenically unsaturated monomers in a powder bed in the presence of polymerization initiators and water while maintaining the powder state, removing the heat of polymerization by distilling off water and circulating the reaction mixture, by a process in which water-insoluble organic peroxides which are liquid at 20.degree. C. are used alone or as a mixture with up to 50% by weight, based on the initiator mixture, of water-soluble, organic azo initiators and/or water-insoluble solid peroxy-containing compounds, as polymerization initiators.